Formation of tools for producing threads with varying pitch

ABSTRACT

A method of forming a threading tool for producing a thread of varying pitch in a work piece by relative rotary and axial movement between the tool and work piece. The tool is constructed to have a series of threading projections including a first series of pre-forming projections for initially producing a partial thread and groove in the work piece, and one or two final projections for reshaping the partial thread to have the varying pitch. The projections may be formed in two or more steps, including a first step of initially shaping both the pre-forming and final projections to an initial form, and a subsequent step of then removing material from one axial side of the pre-forming projections in a relation giving them a reduced axial thickness less than that of the final projection or projections, and to an extent such that, upon relative rotary and axial movement betwee- the tool and work piece, the preforming projections will at all times remain within the axial sectional profile of the thread groove of varying pitch ultimately formed by the final projections.

This application is a continuation-in-part of U.S. patent application Ser. No. 7/904,499 filed Jun. 25, 1992 on "Formation of Threads With Varying Pitch", now U.S. Pat. No. 5,316,520.

The invention relates to the manufacture of tools for forming threads of varying pitch in a work piece.

BACKGROUND OF THE INVENTION

U.S. Pat. Nos. 4,842,464, 4,956,888 and 5,086,532 disclose a novel type of nut having a thread of varying pitch which acts to distribute the axial load applied to the nut more uniformly between the different turns of the thread than is possible with a conventional thread whose pitch does not vary. These patents show methods and apparatus for manufacturing such nuts by employment of lathe type threading tools having a single point which takes a series of cuts in the work piece, or a similar tool having two projections for simultaneously forming opposite side surfaces of the thread. U.S. Pat. No. 5,250,007 shows another method of forming a thread of varying pitch, utilizing a threading assembly consisting of several thread forming elements which rotate and advance axially relative to the work piece essentially in unison, and which also shift axially relative to one another during a threading operation in a manner producing together the desired thread of varying pitch.

SUMMARY OF THE INVENTION

The present invention provides improved methods for manufacturing threading tools capable of producing threads of varying pitch in a simpler and less expensive manner than in the above discussed prior patents. A tool formed in accordance with the invention can produce a thread of varying pitch by a process approximately as simple and inexpensive as conventional tapping or die cutting operations for producing threads of uniform pitch. The tool is also essentially as versatile as conventional taps and dies with respect to the diameter of thread which may be produced.

In manufacturing a threading tool by a method embodying the invention, a series of threading projections are formed on the tool body by relative rotary and axial movement between the tool body and a cutting or shaping element. These projections include a number of pre-forming projections for initially producing a partial thread and groove in a work piece by relative rotary and axial movement between the tool body and work piece, and either one or two final projections operable to give the partial thread and groove a predetermined varying pitch by further rotary and axial movement. During the re-shaping action of the final projection or projections, the tool body and work piece are advanced relative to one another at a varying rate of axial advancement per revolution, to give the thread its desired varying pitch.

The method of formation of the tool includes removing enough material from the pre-forming projections to enable them, upon relative rotary and axial movement between the tool body and a work piece, to always remain within the axial sectional profile of the final thread groove of varying pitch ultimately formed by the final projection or projections to thereby avoid interference by the pre-forming projections with the shape of the final groove.

The pre-forming projections may initially be cut to a thickness slightly greater than their final thickness, with a controlled amount of material then being removed from one or both sides of each projection to give them a reduced size which will not interfere with the groove of varying pitch ultimately produced by the final projection or projections. In one method embodying the invention, both the pre-forming and final projections may initially be cut to the same axial sectional configuration, with material then being removed from a side of the pre-forming projections but not as deeply if at all from the final projection or projections to arrive at the final configuration of the tool. Also, at some point during the tool manufacturing process, surfaces at one side of at least some of the thread forming projections, as viewed in axial section, may be shaped to converge circularly toward and be disposed at a different lead angle than corresponding surfaces at the opposite sides of the projections, to enable the projections to effectively cut a work piece during a threading operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and objects of the invention will be better understood from the following detailed description of the typical embodiments illustrated in the accompanying drawings, in which:

FIG. 1 is a diagrammatic representation of apparatus for producing an internal thread of progressively increasing pitch within a nut body, and including a tapping tool which may be formed by the methods of the present invention;

FIG. 2 is an enlarged transverse section through the tapping tool of FIG. 1, taken on line 2--2 of that figure;

FIG. 3 is a greatly enlarged view taken on line 3--3 of FIG. 2, with the tapping tool shown after it has advanced into the nut body;

FIG. 4 is a view similar to FIG. 3, but showing the thread in the nut body after the tapping tool has moved entirely through the nut body and the thread of increasing pitch has been completely formed;

FIG. 5 is an enlarged fragmentary view corresponding to the left hand portion of FIG. 3, but with the tapping tool illustrated as it appears after it has been advanced rightwardly one full turn beyond the position of FIG. 3;

FIG. 6 is an enlarged fragmentary view showing the right end turn of the completely formed thread of FIG. 4, with illustration on the figure of the cuts of the various pre-forming projections of the tapping tool;

FIG. 7 is an enlarged fragmentary section taken on line 7--7 of FIG. 3;

FIG. 8 is a fragmentary view similar to FIG. 3, and showing a variational form of threading tool;

FIG. 9 is an enlarged diagrammatic view showing the cuts made in the right hand thread groove of FIG. 8 during the first portion of a threading operation utilizing the tool of FIG. 8;

FIG. 10 is a fragmentary view similar to FIGS. 3 and 8 of another variational type of threading tool;

FIG. 11 is a fragmentary view corresponding to a portion of FIG. 1 and showing another form of threading tool;

FIG. 12 represents diagrammatically a process embodying certain features of the invention and utilizing two separate threading tools;

FIG. 13 illustrates diagrammatically a process for manufacturing the threading tool of FIGS. 1 to 7;

FIG. 14 is an enlarged axial section through a portion of the apparatus of FIG. 13 and illustrating three steps in the preferred process of forming the threading tool; and

FIG. 15 is a fragmentary view taken partially on line 15--15 of FIG. 14 and developed circularly about the axis of the tool being formed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates diagrammatically at 10 apparatus embodying the invention for forming an internal thread of progressively increasing pitch within a cylindrical bore 11 in a nut body 12. The thread is produced by moving the nut body and a threading tool 16 both rotatively and axially relative to one another in timed relation. Any type of lathe, threading machine, screw machine or other machine tool capable of producing such relative movement may be employed. For example, the nut body may be held by a chuck 13 of a lathe or other tool, with the chuck being mounted for powered rotation with the nut about axis 14. A tool holder 15 may carry the tapping tool 16, centered about axis 14, and be mounted for controlled movement along axis 14 toward and away from chuck 13. Computer controlled driving mechanism diagrammatically represented at 17 may rotate chuck 13 and the nut about axis 14, and move tool holder 15 and tap 16 along axis 14 in timed relation to the rotation of the nut. The drive connections from mechanism 17 to chuck 13 and tool holder 15 are diagrammatically represented at 18 in FIG. 1.

Instead of rotating the nut and moving the tool 16 axially, these motions may of course be reversed if desired, with tool holder 15 and tap 16 being driven rotatively but not axially, and with the chuck 13 and work piece 12 being shifted axially but not driven rotatively. Also, either the tapping tool or nut may be driven both rotatively and axially, or both may have each of the two types of motion, so long as the desired relative axial and rotary movement between the tapping tool and work piece as discussed hereinafter is attained.

Tapping tool 16 may be formed as a one piece body of tool steel or other hardened material capable of cutting a thread within the interior of nut 12 without damage to the tapping tool. The tapping tool may-have a shape very similar to a conventional thread cutting tap, being elongated in a left to right direction as viewed in FIG. 1, and having a mounting portion 19 adapted to be gripped and held by tool holder 15. Rightwardly of the tool holder 15 in FIG. 1, the tapping tool 16 may have the cross section illustrated in FIG. 2, typically forming four radially outwardly projecting portions 20a, 20b, 20c and 20d carrying outwardly projecting cutting teeth 21, with recesses or flutes 22 formed circularly between the portions 20a, 20b, etc. for reception of lubricant, cooling fluid and cuttings removed from the work piece. In lieu of the arrangement illustrated in FIG. 2, with the four outwardly projecting cutter carrying portions of the tap body, that body may of course have more or less than four such portions as is well known in the threading art. The cross section illustrated in FIG. 2 is uniform along the entire length of the active threading portion of the tapping tool, from its extremity 23 to the location 24 in FIG. 1. Leftwardly of that location, the body of tapping tool 16 may be externally cylindrical, square, or of any other cross sectional configuration capable of being effectively gripped and held by chuck 15. When the thread to be formed in nut body 12 is a right hand thread, portion 20a of tool 16 is the first of the four portions of the tool to engage any particular part of the nut body during a threading operation, followed by portions 20b, 20c and 20d in that order.

The thread cutting projections 21 at the top of FIGS. 1 and 2 are represented in greatly enlarged form in FIG. 3. These projections include a number of pre-forming projections 21c, 21d, 21e, 21f, 21g and 21h which partially shape the thread in the nut body, and two final shaping projections 21a and 21b at the left end of FIG. 3 which give the thread its desired ultimate increasing pitch configuration. To describe first of all the action of the final projections 21a and 21b, these two projections have the thread shaped axial cross section illustrated in FIG. 3, for forming a similarly shaped thread 25 in the interior of nut body 12, as chuck 13 and nut body 12 are rotated about axis 14, and as tool 16 is simultaneously advanced rightwardly along that axis. Thread 25 formed by projections 21a and 21b preferably has the outline configuration of a standard sixty degree thread, defined by a trailing flank surface 26, a leading flank surface 27, and an axially extending crest surface 28. In the position of FIG. 3, surfaces 26 and 27 are being cut to the illustrated shape by a cutting edge 29 at the leading side of projection 21a, and a cutting edge 30 at the trailing side of projection 21b, with these edges 29 and 30 (and the formed surfaces 26 and 27) being disposed at a sixty degree angle to one another. A third cutting edge 31 extending axially between projections 21a and 21b forms the crest surface 28 of the produced thread,

The pre-forming projections 21c through 21h of FIG. 3 are smaller than projections 21a and 21b and act to partially form the thread groove in the nut body before the final shaping process by projections 21a and 21b. In FIG. 3, six such pre-forming projections are typically illustrated, though it will be understood that more than six or less than six may be employed. The first of these projections to contact the work piece during a threading operation is the right end projection 21h, which as illustrated is relatively short radially to make an initial shallow cut in the nut body. Projection. 21g has a somewhat greater radial dimension, to make a deeper cut, and the remaining projections 21f, 21e, 21d, and 21c successively deepen the inter-thread groove in the nut body before the final shaping operation by projections 21a and 21b.

The cutting projections 21a through 21h described above and shown in FIG. 3 are all formed on portion 20a of the tapping tool 16 (see FIG. 2). Each of the other three portions 20b, 20c and 20d of the tapping tool has a series of pre-forming projections corresponding to projections 21c through 21h of FIG. 3, but does not have the final shaping projections 21a and 21b. The projections 21c through 21h of the four portions 20a, 20b, etc. of the tapping tool 16 may be given their progressively increasing radial dimensions by machining these projections to have outer tapering conically curving surfaces 32 centered about axis 14. This conical truncation of the pre-forming projections may be such that the final pre-forming projection 21c of portion 20d of tool 16 (that is, the last preforming projection to engage any particular portion of the nut body before projection 21b contacts that portion of the nut) is not truncated at all and has a full radial height corresponding to that of projections 21a and 21b.

The projections 21c through 21h of portion 20b of the tapping tool are shifted slightly in a leftward direction (leftward as viewed in FIGS. 1 and 3) relative to the corresponding teeth 21c through 21h of portion 20a of the tool because of the ninety degree offset of portions 20a and 20b relative to one another about axis 14. This leftward shift causes each of the pre-forming projections 21c through 21h of portion 20b of the tapping tool to project slightly farther radially outwardly than the corresponding projection of portion 20a as a result of the conical truncation of all of the projections at 32. Similarly the cutting projections of portion 20c are shifted farther to the left in FIG. 3, and have a greater radial dimentsion, than the corresponding projections of portion 20b, and the projections of portion 20d are shifted leftwardly farther to the left, and extend farther out radially than the projections of portion 20c. Thus, the projections 21c through 21h of the four portions of the tapping tool all lie along an essentially helical path enabling the projections of the various portions 20a, 20b, 20c and 20d to sequentially engage and cut any particular portion of the nut body during a threading operation and thereby form an essentially helical thread groove in the nut body in a manner similar to a conventional tapping tool. The helix defined by all of these projections 21c through 21h is of uniform pitch from the right end of the tapping tool through all of these cutting edges. Also, projections 21a and 21b may be considered as lying along essential the same uniform pitch helical path.

As an aid in understanding the preferred relationship between the configuration of cutting projections 21c through 21h of FIG. 3 and the final precise shaping teeth 21a and 21b, FIG. 3 includes reference lines 33c through 33h representing in broken lines a uniform pitch continuation, of the thread pattern defined by projections 21a and 21b. For example, the lines 33c immediately to the right of projection 21b define a thread shaped profile having the same axial sectional outline configuration as projection 21a and projection 21b, and spaced from projection 21b a pitch distance p which is equal to the pitch distance p between projections 21a and 21b. Similarly, the lines or pattern 33d are spaced that same distance from lines 33c, and the other reference lines 33e, 33f, 33g and 33h are all spaced apart the same uniform pitch distance.

The leading cutting edge 34 of each of the projections 21, 21d, 21e, 21f, 21g and 21h preferably coincides with the angularly disposed line 134 defining the leading side of the corresponding broken line reference pattern 33c, 33d, 33e, 33f, 33g or 33h. The cutting edges 35 at the trailing sides of projections 21c, 21d, 21e, 21f, 21g and 21h, however, do not coincide with the angularly disposed lines 36 of the reference patterns 33c, 33d, 33e, etc., but are parallel to lines 36 and spaced rightwardly a substantial distance relative thereto. This leaves a portion 37 of the nut body to the left of each of the cutting projections 21c through 21h, which allows for the slight increase in pitch of the thread ultimately to be formed in the nut body by the final shaping projections 21a and 21b.

To now describe a cycle of operation of the apparatus of FIGS. 1 to 7, assume that cutting tool 16 is positioned within holder 15 as shown in FIG. 1, and is initially in the position of that figure just to the left of nut body 12. The nut body may at the outset have a bore 11 of a diameter corresponding to or slightly less than that of the minor diameter cutting edges 31 of the threading tool. The nose portion 38 of the tapping tool 16 may have a slight conical taper about axis 14, and be a close fit within bore 11 to assist in centering the tapping tool within the nut body. To perform a tapping operation, the drive mechanism 17 is actuated to commence rotation of chuck 13 and nut body 12 about axis 14 at a predetermined rate, and to commence programmed axial movement of tool holder 15 and tapping tool 16 in a rightward direction in predetermined timed relation to the rotation of the nut body. During an initial portion of the threading operation, as tapping tool 16 advances rightwardly from the position of FIG. 1 just outside nut body 12 to the position of FIG. 3, the axial advancement of tapping tool 16 rightwardly relative to nut body 12 may be at a uniform rate, advancing tool 16 exactly the pitch distance p of FIG. 3 during each revolution of the nut body. As a result, the pre-forming cutting projections 21c through 21h of the four portions 20a, 20b, 20c and 20d of the tapping tool progressively form and deepen a helical thread groove of uniform pitch in the nut body. When the FIG. 3 position is reached, the portion of the nut body which is then in contact with projection 21c of portion 20a of the tool is being cut by that projection to a cross section corresponding to that of the illustrated projection 21c. The corresponding projections 21c of the other three portions 20b, 20c and 20d of the tool are at the same time cutting the nut body to a slightly greater radial depth, giving the groove an ultimate major diameter the same as the major diameter of an outer cutting edge 230 of projection 21b.

When the tool reaches a point at which the final shaping teeth 21a and 21b are commencing their cutting operation in the work piece, such as the position illustrated in FIG. 3. or a slightly earlier position, the computer controlled mechanism 17 automatically converts to a changed condition in which, instead of advancing the threading tool rightwardly at a uniform rate per revolution, the tool is advanced rightwardly at a progressively increasing rate per revolution. In a nut of the type disclosed in U.S. Pat. No. 3,842,464, that increase in the rate of advancement, and the resultant increase in pitch in the thread formed in the nut body, are relatively slight, typically being on the order of one tenth of one thousandth of an inch per revolution for certain thread sizes such as 5/8×16. In order to make this advancement visible in the drawings, the advancement has been exaggerated.

FIG. 5 represents the positions of the final cutting projections 21a and 21b after they have advanced through one complete revolution of the nut body from the position of FIG. 3. FIG. 4 shows the completed thread in the nut body after all of the cutting projections 21a through 21h of tool 16 have advanced entirely through the nut. Reference lines 33c, 33d, etc. and the FIG. 3 positions of the cutters have been included in FIG. 4 to assist in understanding the thread cutting action.

When the nut body and tool 16 are in their FIG. 3 positions (or a slightly earlier position if preferred) the rate of relative axial advancement of the threading tool and nut, and the resultant lead angle of the portion 71 of the thread then being formed, correspond to the pitch distance p between projections 21a and 21b. That is, in the FIG. 3 position the tool 16 and nut body are advancing axially relative to one another at an instantaneous rate and lead angle and effective pitch which would advance these parts axially through the distance p in one revolution if the rate of advancement did not increase. In this connection, the "effective pitch" of the relative motion and formed thread at any particular point is defined as the distance which the thread would advance axially from that point during a single revolution if the rate of advancement did not change. The "effective pitch" in the FIG. 3 position is therefore the distance p.

During one revolution of the nut body from the FIG. 3 position, the rate of relative axial advancement of the tool and nut body progressively increases slightly so that, by the time projection 21b reaches its FIG. 5 position, it does not coincide with the previously mentioned reference lines 33c, but is shifted slightly to the right relative thereto. Similarly, the projection 21a does not, at the end of the first revolution, coincide in position with the FIG. 3 position of projection 21b at the commencement of that revolution, but is shifted rightwardly relative thereto (shift exaggerated in FIG. 5). As a result, the trailing cutting edge 30 of projection 21b and the leading cutting edge 29 of projection 21a, and the axially extending edge 31 therebetween, form a second turn 72 of the thread in the nut which is shifted slightly in a rightward direction relative to the uniform pitch reference lines 33c and thus has an increased lead angle and effective pitch greater than the starting pitch p. In FIG. 5, the FIG. 3 position of the leading edge 29 of projection 21 b is represented at 129. The trailing edge 40 of projection 21a is in FIG. 5 spaced slightly from the surface 27 previously formed by edge 30 of the projection 21b.

During the next revolution of the nut body and tapping tool relative to one another, the rate of axial advance of the tapping tool continues to increase at the same uniform rate, so that the third turn 73 of the thread is shifted farther to the right relative to reference patterns 33c and 33d, as represented by the spacing at 78 in FIG. 7. Similarly, the fourth turn 74 is shifted still farther, as represented at 79, and each of the other turns 75, 76, 77 and 78 is shifted farther than the preceding turn (see spacing at 80, 81, 82 and 83). The shift is never great enough, however, to permit the cuts previously made by pre-forming projections 21c through 21h to interfere with the shape of the ultimate increasing pitch groove formed by projections 21a and 21b. This is true because of the reduced width of each projection 21c through 21h as represented at 37 in FIG. 3. The right flank surfaces 183 of the thread turns do not reach the lines 84 of FIG. 4 which define the cuts made in FIG. 3 by the left cutting edges 35 of pre-forming cutters 21c, 21d, etc. The axial dimension of the inter-thread groove in FIG. 4 gradually increases at each turn (in a rightward direction), while the axial cross section and axial thickness of the formed thread itself remain uniform. In this way, the desired thread of progressively increasing pitch is formed in the nut. The thread cutting operation performed by projections 21a and 21b is similar to that performed by the two projections or teeth of the threading tool 54 shown in FIGS. 10 and 11 of U.S. Pat. No. 4,842,464.

All of the various cutting projections 21a, 21b, 21c, etc. of the threading tool 16 are relieved circularly behind their cutting edges in a manner permitting those edges to properly engage and cut the material of the nut body at the different lead angles and effective pitches at which the cutting projections move relative to the nut body during formation of the thread of increasing pitch. This feature is illustrated in FIG. 7, which shows a portion of the cutting projection 21b of FIG. 3 in cross section. The cutting edges of that projection 21b are at the lower end of the projection as viewed in FIG. 7, and include the previously mentioned cutting edge 30 at the left side of projection 21b, a cutting edge 130 at the right side of the projection, and an axially extending cutting edge 230 (visible in FIG. 3 but not in FIG. 7) for forming the root portion of the thread groove in the nut body. These three cutting edges define the thread profile illustrated in FIG. 3, corresponding to that of a standard sixty degree thread. The opposite side surfaces 48 and 49 of cutting projection 21b circularly behind cutting edges 30, 130 and 230 have a similar sixty degree profile approximately the same as the cutting edges, but with the axial thickness of that profile gradually reducing toward the trailing end 50 of the cutting projection, as represented by the fact that the surfaces 48 and 49 gradually converge in FIG. 7 from their leading ends to their trailing ends. This tapering of the cutting projection is such that surface 49 at the right side of the projection is disposed at a lead angle at least as great as, and preferably slightly greater than, the maximum lead angle at which projection 21b and the nut body move relative to one another during a threading operation, that is, the maximum lead angle of the thread formed in the nut body. The maximum lead angle of the thread of course occurs at the right end of the nut in FIG. 3. The opposite side surface 48 of projection 21b is disposed at a lead angle at least as small as, and preferably slightly smaller than, the minimum lead angle at which projection 21b moves relative to the nut body, and the resultant minimum lead angle of the formed groove in the nut body, at the left end of that body. This relationship permits the cutting edges to properly engage and cut the material of the nut body at all times during the thread cutting operation, without interferance surfaces 48 and 49, and in spite of the fact that the rates of axial advancement of the cutting tool and the formed thread per revolution vary during that operation. While FIG. 7 illustrates the tapered shape of only the single cutting projection 21b, it will be understood that all of the other cutting projections are similarly tapered, with the same relationship between the lead angles of their opposite side surfaces and the maximum and minimum lead angles of the tool and nut relative motion and resulting thread.

FIG. 6 illustrates in enlarged form the final configuration of the right hand end portion of the thread groove 41 in the nut body as viewed in FIG. 4. In FIG. 6, the cut made by projection 21h is represented in broken lines at 121h, and the cuts made by projections 21g, 21f, 21e, 21d and 21c are represented in broken lines at 121g, 121f, 121e, 121d and 121c, respectively. Each of these cuts is shifted slightly to the right relative to the preceding cut, by reason of the fact that there is an increase in the rate of axial advancement of the threading tool beyond the FIG. 3 position of that tool. The final cut made by edge 29 of projection 21a, to form the right wall 45 of the thread groove, eliminates the irregularity formed in that wall of the groove resulting from the rightward shift of the cuts made by projections 21c, 21d, etc.

It is particularly noted in FIG. 6, as previously pointed out with respect to FIG. 4, that the reduced axial thicknesses of cutting projections 22c through 22h, and the cuts 121c through 121h made thereby, resulting from the discussed spaced relationship at 37 in FIG. 3, allow those projections and their cuts to at all points along their path of movement relative to the nut body fall within the axial sectional profile of the inter-thread groove 41 of varying pitch ultimately formed in the work piece, None of the cuts 121c through 121h of FIG. 6 is wide enough to interfere with wall 44 or 45 or high enough to interfere with upper surface 46 of the groove, thus permitting the final projections 21a and 21b to give the thread its ultimate shape. This is true at all points along the entire generally helical extent of the formed thread. A drawing along the lines of FIG. 6 could be produced for any desired point along the length of the thread.

In the threading process as thus far described, it is assumed that the threading tool 16 is advancing axially relative to nut body 12 at a uniform rate per revolution until it reaches the position illustrated in FIG. 3, and is then advanced at a progressively and uniformly increasing rate per revolution until all of the cutting projections have moved entirely through the nut body and completed their threading operation. As a variation of the invention, the threading tool may be advanced at an increasing rate through the entire threading process, that is, from a point prior to initial engagement of the cutting tool with the left end of the nut body to the point at which all of the cutting projections of the tool have moved through the nut body and out of contact with it at its right side. FIG. 8 illustrates a tool 51 which may be utilized in this manner. The threading tool 51 may be identical with tapping tool 16 as illustrated in FIG. 3 except that the six preforming projections 52c, 52d, 52e, 52f, 52g and 52h of FIG. 8 (corresponding to projections 21c, 21d, 21e, 21f, 21g and 21h of FIG. 3) are cut away slightly at their leading sides (right sides in FIG. 8) as well as their trailing sides. To assist in understanding this feature, FIG. 8 includes the same uniform pitch thread shaped reference lines or patterns 33c, 33d, 33e, 33f, 33g and 33h as in FIG. 3. In FIG. 3, the leading cutting edge 34 of each of the pre-forming projections 21c, 21d, etc. coincides with the angularly disposed line 134 defining the leading side of the corresponding reference pattern 33c, 33d, etc. In FIG. 8, the leading cutting edges 53 (corresponding to edges 34 of FIG. 3) do not coincide with the inclined lines 134, but instead are parallel to those lines and spaced leftwardly therefrom, in a manner similar to the previously described spaced relationship at the left sides of the cutting projections, as discussed in connection with FIG. 3. This relief of the cutting projections at their right sides in FIG. 8 enables those projections, during axial advancement of the tool before reaching the FIG. 3 position, to at all times remain entirely within the axial sectional profile of the increasing pitch thread ultimately formed in the nut body, and thus prevent the cutting projections 52c , 52d, etc. from interfering with proper formation of the opposite side walls of the thread groove. It will of course be understood that the left end of the threading tool 51 of FIG. 8 includes two thread cutting projections identical with the projections 21a and 21b of FIG. 3 and having the same relationship to reference patterns 33c, 33d, etc. as in FIG. 3, The projections 21a and 21b have been omitted in FIG. 8 to simplify the drawing.

FIG. 9 illustrated diagrammatically the manner in which the axial advancement of threading tool 51 of FIG. 8 is controlled to maintain the various pre-forming cutting projections 52c, 52d, etc. within the profile of the ultimate thread groove formed in the nut body during the initial advancement of the tool up to a position corresponding to that of FIG. 3. When the tool of FIGS. 8 and 9 is in a position corresponding to or slightly to the left of that shown in FIG. 3, in which position the final thread shaping projections 21a and 21b are commencing their threading operation to produce the ultimate thread of increasing pitch, the tool 51 is at that position being advanced axially relative to the nut at a lead angle and effective pitch corresponding to pitch p. Consequently, the portion of the thread then being formed in the nut has that lead angle and effective pitch. When tool 51 is in a position leftwardly of the FIG. 3 position, such as the position of FIG. 1 just prior to engagement of the threading tool with the nut body, the tool is being advanced axially in a rightward direction at a rate and lead angle and effective pitch less than the rate and lead angle and effective pitch in the FIG. 3 position. From the position of FIG. 1 to a position corresponding to that of FIG. 3, the rate of axial advancement of the tool 51 per revolution gradually increases, preferably at a uniform rate. Beyond the position corresponding to FIG. 3, the rate of advancement continues to increase, preferably at that same uniform rate per revolution, until all of the cutting edges have moved rightwardly beyond the right end of the nut body. During the portion of this advancement up to the point at which the tool reaches the FIG. 3 position, the various cutting edges 52c, 52d, etc. make cuts in the nut body which are slightly shifted relative to one another, as represented in FIG. 9. The thread groove 54 illustrated in that Figure may be considered as corresponding to the groove 154 which is nearest the left end of the nut body in FIG. 4.

When the smallest cutting projection 52h of FIG. 8 is in contact with the nut body 12, at the location illustrated in FIG. 9, projection 52h makes the cut identified by that number in FIG. 9. Because of the slow rate of advancement of the threading tool at that point, the cut 52h is shifted rightwardly, with the result that its right edge 55 is near but does not reach the plane in which the right hand wall 56 of the ultimate increasing pitch thread groove 54 is to be formed by the final two thread shaping projections 21a and 21b. If projection 51h were not cut away at its right side as illustrated in FIG. 8, the right portion of that projection would extend rightwardly beyond the planar surface 56 and prevent final formation of that surface at a proper location. In similar manner, the projections 52g, 52f, 52e, 52d and 52c make sequential cuts at the locations represented in FIG. 9. Because of the slow though progressively increasing rate of advancement of the tool, each of these successive cuts is shifted slightly to the left of the preceding one as illustrated in FIG. 9. However, all of these cuts in their entirety always remain within the profile of the groove 54 ultimately to be formed by the final shaping projections 21a and 21b. The same is true at all points rightwardly beyond the location represented in FIG. 9, with the cuts made by the different cutting projections remaining within the profile of the final groove to be formed at all locations. After the tool reaches a position corresponding to that of FIG. 3, the cutting action of the tool 51 of FIG. 8 is essentially the same as has been described in connection with FIG. 3. It will of course be understood that the tool of FIG. 8 may if preferred be operated in accordance with the timing cycle described in connection with the arrangement shown in FIG. 3, with the tool 51 being advanced axially at a uniform rate per cycle until a position corresponding to FIG. 3 is reached, and then being advanced at a uniformly increasing rate while the final shaping projections 21a and 21b are acting on the nut body.

FIG. 10 illustrates fragmentarily another variational arrangement, in which the threading tool 57 may be identical with that shown in FIG. 3 except that the various pre-forming projections represented at 21c, 21d, 21e, 21f, 21g and 21h in FIG. 3 are shaped slightly differently at their trailing sides. FIG. 10 shows only four of these projections, identified by the numbers 58c, 58d, 53g and 53h, with the intermediate projections (corresponding to 21e and 21f of FIG. 3) being omitted to simplify the drawing. The left edge 59 of the smallest of these projections 58h may be substantially the same as the left edge of projection 21h in FIG. 3, being parallel to and spaced a substantial distance rightwardly of the adjacent portion of reference line 33h. The next larger projection 58g has its left hand cutting edge 60 slightly closer to its adjacent reference line 33g, but still parallel thereto. The cutting projections leftwardly of projection 58g similarly have edges 61 which are parallel to but move progressively closer to the corresponding reference lines. This change may be a gradual and progressive change, with the spacing ultimately disappearing and the cutting edge almost coinciding with the reference line at the left side of cutter 58c, the last pre-forming cutter which engages the nut body just prior to its contact with the first of the final shaping projections 21b.

In utilizing the tool of FIG. 10, the timing procedure described in connection with FIGS. 3 and 4 may be utilized, to advance the tool 57 axially at a uniform rate per revolution up to the position illustrated in FIG. 3 with a progressive and uniform increase in the rate of axial advancement per revolution beyond that FIG. 3 position. The progressive increase in width of the cutaway regions at the left sides of cutters 58c through 58h is permissible because the final shaping projections 21a and 21b are shifted only slightly relative to reference lines 33c, etc. near the left end of the nut body and are shifted much farther relative to the reference lines as the cutters advance toward the right end of the nut.

FIG. 11 shows another form of threading tool 62 which may be identical with the tool 16 of FIGS. 1 to 7 except that the final thread shaping projections 63 and 64, corresponding to projections 21a and 21b of FIG. 3, are spaced axially from all of the other projections 65, corresponding to projections 21c, 21d, 21e, 21f, 21g and 21h of FIG. 3, with cylindrically curved external surfaces 66 formed on the tool axially between the spaced projections and at a diameter not greater than the minor diameter of the cutting projections and the thread formed thereby. The space between projections 64 and 65 is long enough to permit the pre-forming projections 65 to all advance entirely through the nut body 67 and complete formation of their uniform pitch thread therein before projections 63 and 64 first contact the nut. The tool 62 is advanced axially at a uniform rate per revolution while projections 65 perform their pre-forming operation, and is advanced at a progressively and uniformly increasing rate per revolution while projections 63 and 64 perform their final shaping operation, as in FIG. 3.

Some but not all of the advantages of the invention can be attained utilizing two separate threading tools 68 and 69 as shown in FIG. 12. The first of these tools has several circularly spaced series of thread cutting pre-forming projections 70 which may be the same as projections 21c, 21d, 21e, 21f, 21g and 21h of FIG. 3. Tool 68 is advanced both rotatively and axially at a preferably uniform rate per revolution, relative to the nut body 12, from the full line position to the broken line position of FIG. 12, so that projections 70 form a partial thread of uniform pitch in essentially the same manner as in FIG. 3. The nut is then moved to a position opposite tool 60, which has two projections 170 corresponding to projections 21a and 21b of FIG. 3, and which is advanced rotatively and axially relative to and through the nut body at a progressively and uniformly increasing pitch, in correspondence with the movement of projections 21a and 21b in FIG. 3 relative to the nut, to give the thread its final increasing pitch configuration.

The projections 70 of threading tool 68, instead of being shaped in correspondence with projections 21c, 21d, etc. of FIG. 3, may have any other shape capable of forming a partial thread groove which will be contained entirely within the space defined by the inter-thread groove of increasing pitch ultimately formed by projections 170 of tool 69. For example, projections 70 may correspond to projections 52c through 52h of FIG. 8, or projections 58c through 58h of FIG. 10. In this connection, it is noted that projections 52c through 52h of FIG. 8 have an axial sectional cutting profile which is essentially the same as an undersize conventional tap for a standard 60 degree thread. Consequently, in the process of FIG. 12, the pre-forming tool 68 may be such a standard tapping tool, of a size designed for tapping of a thread of a pitch diameter slightly smaller than that of the increasing pitch thread ultimately to be formed in nut body. This tool will then form a thread groove in the nut body having a uniform pitch and of reduced axial width because of the reduced pitch diameter of the tool to lie entirely within the profile of the final increasing pitch inter-thread groove.

All of the forms of the invention thus far described produce a thread whose axial thickness and axial cross section remain uniform along the entire length of the thread, with the groove between successive turns of the thread progressively increasing in axial width in correspondence with the increase in pitch of the thread. The uniform cross section of the thread results because in each instance two thread cutting projections, such as projections 21a and 21b of FIG. 3, simultaneously form opposite side surfaces of the thread, such as surfaces 26 and 27 in FIG. 3. It is contemplated, however, that any of the forms of the invention may be adapted to produce a thread whose axial thickness increases progressively while the groove between turns of the thread is of uniform axial width. This may be accomplished with the FIG. 3 tool, for example, by omitting its projection 21a, and otherwise constructing and operating the tool as previously described. Projection 21b then acts by itself as a single point threading tool to define the unchanging shape of the groove and cause the increase in thickness of the thread in correspondence with the increase in the rate of advancement of the tool. The same result can be attained by omitting the projection corresponding to projection 21a in any of the other forms of the invention. In each instance, as in the other procedures previously described, the cuts made by pre-forming projections 21c through 21h always remain within the axial profile of the thread groove of increasing pitch ultimately formed by projection 21b, and thus do not interfere with the proper configuration of that groove.

FIGS. 13 through 15 illustrate diagrammatically a process and apparatus for manufacturing the thread tapping tool 16 of FIGS. 1 to 7. It will be apparent that similar processes may be employed for producing the other forms of tapping tools embodying the invention. With reference first to FIG. 13, there is illustrated at 83 in that Figure a metal body from which the tapping tool 16 of FIGS. 1 to 7 is to be formed. It is assumed in FIG. 13 that body 83 has already been machined or otherwise formed to the condition shown in FIG. 1 except for formation of the thread forming projections 21a through 21h on the body. Mounting portion 19 at the left end of body 83 is held by a chuck 84 which may rotate tool body 83 about an axis 85 under the control of a computer actuated drive mechanism 86. A thread forming tool 87 may be carried by a tool holder 88 which is power driven axially, that is, parallel to axis 85, under the control of computer actuated drive mechanism 86, and in timed relation to the rotation of chuck 84 and tool body 83. Alternatively, any other type of drive arrangement may be utilized which is capable of producing timed relative rotary and axial movement between body 83 and cutter 87.

The portion 89 of body 83 which protrudes from chuck 84 may have a cross section corresponding essentially to that illustrated in FIG. 2 except that the outer extremities of the four radially outwardly projecting portions 20a through 20d are unthreaded and have outer cylindrically curved surfaces 90 centered about axis 85 and of a diameter equal to the major diameter of the projections 21a, 21b, etc., through 21h ultimately to be provided on portions 20a, etc. of body 83.

Element 87 may be a conventional thread cutting tool, mounted in holder 88 to project radially inwardly toward axis 85, and having an inner tip portion 91 (FIG. 14) shaped to have an axial sectional profile corresponding to that of a standard sixty degree thread, and defined by cutting edges 92 and 93 at opposite sides of the cutter disposed at a sixty degree angle with respect to one another. A third cutting edge 94 extending axially between the extremities of edges 92 and 93 forms axially extending root surfaces 95 in body 83. In lieu of the cutter illustrated in FIG. 14, element 87 may in certain instances be another type of thread forming device, such as a thread grinding wheel, thread roller, or the like.

FIGS. 14 and 15 illustrate diagrammatically three successive steps which may be performed by thread cutting tool 87 in the outer surfaces of portions 20a, 20b, etc. of body 83. After completion of these three steps, a fourth step may be performed by cutter 87 or another cutter to truncate the pre-forming projections 21c through 21h as represented at 32 in FIG. 3, or if desired that truncating step may be performed before the three steps illustrated in FIGS. 14 and 15, or between any two of those steps.

The first of the three steps is to form a uniform pitch sixty degree thread in the outer surfaces of body 83 by rotating that body and simultaneously advancing tool holder 83 and thread cutting tool 87 axially (preferably leftwardly in FIG. 14) relative to body 83 at a uniform rate of advancement per revolution corresponding to pitch p of FIG. 3. This step is illustrated by the broken line representation of cutter 87 at 87' in FIG. 14. During such formation of the thread shaped projections on the outer surfaces of the body 83, the previously mentioned cutting edge 92 of cutter 87 forms inclined flank surfaces 96 at the rightsides of those projections, while the opposite cutting edge 93 of cutter 87 forms surfaces 97 at the trailing sides of pre-forming projections 21c, 21d, 21e, 21f, 21g and 21h, and surfaces 197 at the left sides of the two final shaping projections 21a and 21b in the left portion of FIG. 14. This cutting action thus initially forms all of the projections 21a through 21h to be of the same axial sectional profile and at uniform spacing as in a standard thread of pitch p. That configuration may of course be produced by a series of successive cuts in body 83, with element 87 being shifted slightly radially inwardly between the different cuts to progressively deepen the thread groove formed in body 83. Leftwardly beyond projection 21a at the left end of FIG. 14, cutter 87 may be controlled to form a cylindrical surface 198 of a diameter corresponding to minor diameter surfaces 95 at the roots of projections 21a, 21b, etc.

The second step of the process may be to advance cutter 87 and tool body 83 through another cycle of relative rotary and axial movement, with the cutter preferably moving leftwardly from the right end of element 83, in a relation removing material from the left side flank surfaces of preforming projections 21c through 21h, but not the final shaping projections 21a and 21b. The position of the threading tool 87 relative to the thread groove during this step is illustrated by the full line representation of cutter 87 in FIG. 14. In that position, the cutting edge 93 at the right side of element 87, which in the previous step had formed flank surfaces at the locations 97 in FIG. 14, acts to form surfaces 297 at the left sides of the projections. These surfaces 297 will ultimately serve as the inclined left edges 35 of the pre-forming projections of FIG. 3, If the tool to be formed is of the type shown in FIG. 3, the rate of leftward axial advancement of element 87 per revolution of the body 83 while the cutter is in contact with pre-forming projections 21c through 21h is the same during this second step of the process as during the first step, but with the axial advancement slightly delayed or offset to the right with respect to the cycle followed in the first step, so that flank surfaces 297 are spaced rightwardly of the initially formed surfaces 97 the same distance for each of the pre-forming projections 21c through 21h. This second step may if desired be halted before element 87 reaches the two final shaping projections 21a and 21b on portion 20a of Body 83, so that these projections 21a and 21b are left with a full thread profile as in FIG. 3. Alternatively, element 87 may continue its movement past projections 21a and 21b with the same timing as in the first step, to follow the same path relative to body 83 as in the first step and in that way leave projections 21a and 21b with their full thread profile.

If the tool to be formed is of the type shown in FIG. 10, rather than that of FIG. 3, the rate of leftward axial advancement of element 87 per revolution of body 83 may be slightly greater while cutter 87 is in contact with projections 21c through 21h during the second step than during the first step, to progressively reduce the spacing between the left flank surfaces formed in the second step and surfaces 97 formed in the first step, and thus gradually and progressively reduce the amount of material removed from the left flanks of the pre-forming projections 21c through 21h in the second step. The resulting gradual leftward shift of the left flank surfaces relative to the positions of the surfaces 297 in FIG. 14 is represented in FIG. 10 by the gradual reduction in spacing between edges 59, 60 and 61 of FIG. 10 and the reference lines 33h, 33g, etc. of that Figure. That spacing is reduced to zero as the cutter 87 reaches projection 21b so that the position of the cutter at that point in step two coincides with the position of the cutter at the corresponding point in step one. The cutter may then be advanced past the two final shaping projections 21a and 21b at a uniform rate corresponding to that of step one, to follow exactly the same path past projections 21a and 21b as in step one, and thus leave those projections unchanged by step two. In lieu of such movement past projections 21a and 21b, the cutter 87 may of course be halted before reaching those final projections.

A third step in the process of FIGS. 13 to 15 may be performed with cutter 87 again in a position such as that represented in broken lines at 87', to further shape the right hand flank surfaces 96 of all of the projections 21a through 21h, but with the rate of axial advancement of tool 87 per revolution of body 83 being alternately increased slightly and decreased slightly through the entire machining operation to produce surfaces 49 of increased lead angle on the various projections (see FIGS. 7 and 15).

FIG. 15 is a developed view representing diagrammatically the cuts which are made during step three on preforming projections 21c through 21h of the various portions 20a, 20b, 20c and 20d of body 83. The lower portion of FIG. 15 shows two of the tapping projections 21e and 21f on portion 20a of body 83, as viewed on line 15--15 of FIG. 14. Above these projections in FIG. 15, the two corresponding projections 21e and 21f of portion 20b of the tapping tool are illustrated, and still higher the two projections 21e and 21f of portion 20c are shown. The view is thus developed circularly about the axis 85 of body 83 and about the corresponding axis 14 of the finished tool illustrated in FIG. 1. The fourth portion 20d of the tapping tool body has a similar pair of projections 21e and 21f, which have been omitted from FIG. 15 to simplify the drawing.

The broken lines 99 of FIG. 15 represent the cuts made by tool 87 in the second step of the above discussed process, in which the cutter is positioned as shown in full lines at 87 in FIG. 14, that is, while the right hand cutting edges 93 of element 87 are forming surfaces 297 at the left sides of projections 21c through 21h of the four portions 20a, 20b, 20c and 20d of the tapping tool. Since the rate of axial advancement of tool 87 per revolution of body 83 is uniform during formation of the surfaces 297, the lines 99 appear as straight lines in the developed view FIG. 15. The broken lines 100 in FIG. 15 represent the final cuts made on surfaces 96 at the right sides of projections 21a through 21h in the third step of the process. Because the rate of axial advancement of element 87 per revolution of body 83 alternately increases and decreases during this step of the process, the lines 100 appear as axially oscillating or waving lines in FIG. 15. While cutter 87 is in engagement with the right side of each of the projections 21a through 21h of the four portions of the tool body 83, the rate of axial advancement of tool 87 per revolution of body 83 is slightly greater than the rate utilized during the first step of the process in forming the uniform pitch projections at pitch p, so that surfaces 49 at the right sides of the projections (see also FIG. 7) are given a greater axial lead than surfaces 297 at the left sides of those projections. The axial lead of surfaces 49 is as previously discussed at least as great as and preferably slight greater than the maximum lead angle of the varying pitch thread to be formed by tapping tool 16 in nut 12 of FIG. 1. After tool 87 has cut this increased lead angle surface 49 on one of the projections, say for example projection 21f of portion 20a in the lower portion of FIG. 15, the computer controlled drive mechanism 86 acts to automatically slightly reduce the rate of axial advancement of tool 87 per revolution of body 83, to a rate slightly less than that associated with pitch p, as represented by a slight curving of the broken line 100 at 101 in FIG. 15. That reduction in the rate of advancement is enough to properly position cutter 87 for engagement at 102 with the next successive projection to be shaped (projection 21f of portion 20b of the tapping tool), with the rate of advancement then increasing again to form the increased lead surface 49 at the right side of this second projection. The rate of axial advancement is alternately increased and decreased in this same way as cutter 87 successively engages and forms increased lead surfaces 49 at the right sides of all of the projections 21a through 21h of portion 20a of the tool, and all of the projections 21c through 21h of portions 20b, 20c and 20d of the tapping tool. Portions 21b, 21c and 21d of course do not have the final shaping projections 21a and 21b. The outer surfaces of these three portions of the tool may be ground away or otherwise machined or formed to have continuations of the cylindrical surface 198 at the locations at which projections 21a and 21b of these three portions of the tool would otherwise be formed.

It is contemplated that the three steps discussed above may be performed in a different order than has been described or some of those steps may be combined into a single step or omitted, or altered or supplemented by other steps. For example, in producing the tool of FIG. 3, the first two steps may be combined, with the tool 87 acting during a continuous cycle of axial advancement from right to left in FIG. 14 to first form surfaces 297 on pre-forming projections 21c through 21h, and then form surfaces 197 on projections 21a and 21b. During the first portion of this cycle of advancement, cutter 87 is in a position relative to the groove corresponding to the full line position of element 87 in FIG. 14, to form surfaces 297 on projections 21c through 21h. After cutter 87 moves out of engagement with the last of the pre-forming projections, the rate of axial advancement is increased long enough to shift the cutter leftwardly for proper formation of surfaces 197 on projections 21a and 21b. During actual formation of those surfaces 197, the rate of axial advancement per revolution is returned to the same rate as during formation of surfaces 297. It will of course be understood that several passes of the tool may be made to progressively deepen a series of such continuous cuts made in the tapping tool, with the cutter 87 being advanced leftwardly at an increased rate during each of these passes after leaving the last of the pre-forming projections 21c through 21h, to shift the cutter as discussed.

The first two steps may be similarly combined in forming the tool of FIG. 10, with tool 87 acting during a continuous cycle of leftward advancement to first form surfaces 59, 60 and 61 of FIG. 10 in the manner previously discussed, and then on the same cycle form the final projections 21a and 21b. During the formation of surfaces 59, 60 and 61, the tool advances at a slightly greater rate than during the formation of the final projections 21a and 21b.

After either of the above discussed combined processes of producing all of the left side flank surfaces 297 and 197 of FIG. 14, or 59, 60, 61 etc. of FIG. 10, in a single procedure, the surfaces 49 of increased lead at the opposite sides of all of the projections may then be formed by a final cutting operation in which the rate of axial advancement of element 87 per revolution is alternately increased and decreased as represented by broken lines 100 of FIG. 15. As a further possible variation, this process in which surfaces 49 are formed, by an oscillating or waving motion, may itself be the first step of the procedure, with the opposite flank surfaces 297 and 197 (or 59, 60, 61 etc.) at the trailing sides of the projections being formed subsequently either in one or two steps as desired.

As a further variation, it is contemplated that during the intervals when cutter 87 is cutting the left flank surfaces 197 and 297 of projections 21a through. 21h, (or left flank surfaces 59, 60, 61 etc. of the FIG. 10 form), cutter 87 may be advanced axially at a rate slightly less than that associated with the pitch p of FIG. 3, to give the flank surfaces at 197 and 297 (or 59, 60, 61 etc.) a lead angle slightly less than that associated with pitch p, as represented somewhat diagrammatically by the surface 48 of FIG. 7, thus assuring a proper cutting action by the edge 30 of each projection. This effect can be attained by alternately increasing and decreasing the rate of axial advancement of cutter 87 in a manner similar to that discussed in detail above in connection with surfaces 49 of FIG. 15. Thus, the broken lines 99 of FIG. 15 may have a waving configuration similar to but the opposite of the waving configuration of lines 100.

The outer conically tapered surfaces 32 (FIG. 3) of pre-forming projections 21c through 21h may be formed on those projections by machining body 83 conically either before or after the various cutting projections have been formed, or at any intermediate point in the process. Also, the axial recesses 22 in the tool (see FIG. 2) may be formed in body 83 before the projections are formed, after they have been formed, or at any other point.

To describe somewhat more specifically a particular presently preferred two step process for producing the arrangement of FIG. 10, the cutter 87 may in one of the two steps of that process form all of the right side flank surfaces of all of the projections 58a through 58h, and in the other step form all of the left side flank surfaces of all of the projections. The two steps may be performed in either order, that is, with either step first, and each step may include a series of progressively deepening cuts. During each of these cuts, the cutter advances from right to left, to successively form surfaces on projections 58h, 58g, etc. through 58a.

During each cut in forming the right hand flank surfaces, the rate of axial advancement of the cutter per revolution is alternately slightly increased and slightly decreased to follow the waving line 100 of FIG. 15, and thereby produce the surfaces 49 of FIGS. 7 and 15 on each of the projections. The variation in the rate of advancement is such as to give the surface 49 of each projection a lead angle slightly greater than the maximum lead angle at which the tool body 57 and its projections are to be advanced relative to a nut body 12 during a threading operation, that is, the maximum lead angle of the thread formed in the nut body. During each cut in forming the left hand flank surfaces, the rate of axial advancement of the cutter relative to the nut body may be considered as varying in accordance with the summation of two components as follows. As a first of these components, the cutter may be considered as advancing at a basic rate slightly greater than that corresponding to pitch p, to shift surfaces 59, 60 etc. gradually leftwardly as illustrated in FIG. 10, with the basic rate of advancement being reduced to a rate equaling that of pitch p in forming the left flank surfaces of the final two projections (21a and 21b of FIG. 3). Superimposed on this basic rate of advancement, the second component may introduce a slight oscillation alternately gradually increasing and decreasing the rate of advancement, to slightly reduce the lead angle of the left flank surfaces as represented by the angularity of surface 48 of FIG. 7. This variation in the rate of advancement is such as to give the surface 48 at the left side of each projection a lead angle slightly less than the minimum lead angle at which the tool body and its projections move relative to the nut body during a threading operation, and the resultant minimum lead angle of the formed groove in the nut body. As a result of the converging angularity of surfaces 48 and 49 of all of the projections, those surfaces are, during a threading operation, always relieved slightly away from the thread flank surfaces being cut by edges 30 and 31, so that only those edges engage the material of the nut, in a manner enabling the edges to make optimum cuts in the nut body.

While certain specific embodiments of the present invention have been disclosed as typical, the invention is not limited to these particular forms, but rather is applicable broadly to all such variations as fall within the scope of the appended claims. For example, the teachings of the invention may be applied to formation of a thread which varies in pitch in accordance with any desired pattern of change, not necessarily the described simple uniform increase in pitch. The invention may also produce an external rather than internal thread of increasing or otherwise varying pitch, by a die having cutting edges similar to those illustrated in FIG. 3 or others of the figures but projecting radially inwardly to form the external thread. It is also contemplated that the thread forming projections or portions of the tools embodying the invention may in some instances be designed to forceably deform or reshape the material of the work piece to the desired configuration of a thread and groove of varying pitch without actually cutting the material and in a manner similar to a thread rolling operation. 

I claim:
 1. The method of producing a threading tool that comprises:forming on a tool body, by relative rotary and axial movement between the tool body and tool shaping means, a series of pre-forming projections for initially producing a partially formed thread and groove in a work piece by relative rotary and axial movement between the tool body and work piece, and either one final projection or two final projections operable by further relative rotary and axial movement of the tool body and work piece at a varying rate of axial advancement per revolution to simultaneously engage corresponding portions of two flank surfaces at opposite sides of a portion of said partially formed thread or at opposite sides of a portion of said partially formed groove, and to reshape said corresponding portions of both of said flank surfaces simultaneously to a predetermined varying pitch, thereby converting said partially formed thread and groove to a final thread and final thread groove of varying pitch having a predetermined axial sectional profile; said method including making said pre-forming projections small enough, during said formation thereof, to enable said pre-forming projections, upon relative rotary and axial movement between said body and work piece at said varying rate in forming a thread of varying pitch, to remain within the axial sectional profile of the final thread groove of varying pitch ultimately formed by said final projection or projections, and thereby avoid interference with said simultaneous reshaping of said flank surfaces to said varying pitch by said final projection or projections.
 2. The method of claim 1, including first producing said pre-forming projections with excessive axial thicknesses which would not remain within the axial sectional profile of said final thread groove of varying pitch, and then removing material from said pre-forming projections to reduce their axial thicknesses.
 3. The method of claim 1, in which said forming of the projections includes initially shaping the material of both the pre-forming and final projections to have similar configurations at a predetermined axial side of the pre-forming and final projections, and then removing material from said side of the pre-forming projections but not as deeply if at all from the corresponding side of the final projection or projections, and to an extent enabling the pre-forming projections to at all times remain within the axial sectional profile of said final thread groove of varying pitch during a threading operation.
 4. The method of claim 3, in which said material removed from said pre-forming projections is removed from sides thereof which are trailing sides in a threading operation.
 5. The method of claim 1, in which said forming of the projections includes initially producing both the preforming and final projections to have a same axial sectional profile, and then removing material from a side of the pre-forming projections but not as deeply if at all from the corresponding side of the final projection or projections to avoid interference by the pre-forming projections with the shape of the thread and groove of varying pitch formed by the final projection or projections.
 6. The method of claim 1, in which said forming of the projections includes shaping surfaces of a plurality of the projections at a first axial side thereof, by relative rotary and axial movement of the tool body and shaping means, in a relation causing said surfaces to converge toward, and be disposed at a different lead angle than, surfaces at the opposite axial side thereof.
 7. The method of claim 6, in which said surfaces are given said different lead angle by alternately increasing and decreasing slightly a rate of relative axial advancement per revolution of the tool body and shaping means during formation of said surfaces.
 8. The method of claim 1, in which there are two of said final projections to simultaneously form opposite side surfaces of said thread of varying pitch in a work piece.
 9. The method of claim 1, including first producing said pre-forming projections with excessive axial thicknesses which would not remain within the axial sectional profile of said final thread groove of varying pitch, and then removing material from said pre-forming projections to reduce their axial thicknesses, said forming of the projections including shaping surfaces of a plurality of said projections at at least one axial side thereof by relative rotary and axial movement of the tool body and shaping means at alternately increasing and decreasing rates of axial advancement per revolution in a relation causing said surfaces to converge toward, and be disposed at a different lead angle than, surfaces at the opposite side of the same projections.
 10. The method of claim 9, including shaping said pre-forming projections so that, as they approach said final projection or projections, each successive pre-forming projection has an axial thickness slightly greater than the preceding pre-forming projection.
 11. The method of claim 1, including shaping said pre-forming projections so that, as they approach said final projection or projections, each successive pre-forming projection has an axial thickness slightly greater than the preceding pre-forming projection.
 12. The method of claim 1, in which said forming of the projections includes making a series of deepening cuts on the tool body sequentially forming flank surfaces at one axial side of said pre-forming and final projections, and making a different series of deepening cuts in the tool body sequentially forming flank surfaces at the other axial side of said pre-forming projections.
 13. The method of claim 12, including alternately increasing and decreasing the rate of relative axial advancement per revolution of said tool body-and shaping means, during one of said series of cuts, in a relation causing the flank surfaces at one axial side of the projections to converge toward, and be disposed at a different lead angle than, the flank surfaces at the opposite axial side of the projections.
 14. The method of claim 13, including controlling one of said series of cuts so that, as said pre-forming projections approach said final projection or projections, each successive pre-forming projection has an axial thickness slightly greater than the preceding pre-forming projection.
 15. The method of claim 12, including controlling one of said series of cuts so that, as said pre-forming projections approach said final projection or projections, each successive pre-forming projection has an axial thickness slightly greater than the preceding pre-forming projection. 